Brake actuator mechanisms of the prior art have adjustment devices that allow the brake clearance to be precisely adjusted manually from a point exterior to the brake while the brake is installed on the vehicle. The adjustment devices have structures, called back-off tools, that are designed to break at a lower torque rating than any other internal component of the brake actuator mechanism. Such devices are important because they break before other internal components are damaged during an improper adjustment procedure. The internal components of the brake actuator mechanism are expensive and/or difficult to replace, thus they require such protection.
Various prior art devices are known to prevent improper adjustment of brake mechanisms and/or facilitate proper adjustment of brake mechanisms. For example, in both U.S. Pat. Nos. 4,478,316 and 4,478,317 a slack adjuster is taught having clutch means between the adjusting sleeve and the housing. The clutch means includes a helical spring that extends around the sleeve. The clutch means is used to prevent clockwise rotation.
U.S. Pat. No. 4,394,890 provides for an automatic slack adjuster having a force limiting clutch that is wound such that when transferring clockwise torque from the intermediate sleeve to the driven sleeve, its torque transmitting capability is greater than that required to rotate a nut member when the friction linings are not in contact with the disc, but less than the torque required to rotate the nut member if the friction linings are in contact with the disc. A driving sleeve is connected to the intermediate sleeve by another clutch. The clutch is designed to transmit all of the torque developed by the driving sleeve in one direction of rotation to the intermediate sleeve, but will transmit very little of the torque developed by the driving sleeve to the intermediate sleeve in the other direction of rotation.
U.S. Pat. No. 4,399,894 teaches a push rod slack adjuster having a disc clutch drive. The disc clutch drive is comprised of a pair of clutch plates which have a tanged connection to an actuating shaft. A compression spring bears against one of the clutch plates. The spring presets the pressure existing between the various clutch plates and determines the torque at which this clutch will slip.
The prior art devices briefly discussed above, and those known to those skilled in the art, have several disadvantages. First, if the back-off tool in the adjustment device breaks under an excessive torque, although it protected the internal components of the mechanism, the back-off tool still must be replaced prior to completing maintenance on the brake. Replacing a broken back-off tool increases the brake maintenance cost.
Second, an individual performing brake maintenance may not know the back-off tool is broken, if they do know, they may forget to replace it during maintenance. A broken back-off tool can cause damage to the brake mechanism.
In light of at least the above-disadvantages, it would be advantageous to have a device that is permanently installed in the brake actuator mechanism that continuously provides overload protection to the brake actuator mechanism.